It has been found that even after only a relatively short time in operation, lubricants (paraffins, bobbin oil and so forth) sticking to the surface of the ongoing yarn can lead to deposits on the brake disks. If dirt particles or fluff are embedded in these deposits, the result is a sticky, pasty composition that increasingly invades the area between the brake disks or plates. This ever-increasing accumulation during operation spreads the brake disks or plates locally apart over time, making them less and less capable of performing their braking function on the yarn passing through them. Irregular braking action also results, leading to undesired fluctuations in yarn tension. The mobility of the brake disks or plates is hindered as well, allowing the yarn traveling between them to begin cutting into the braking surfaces of the brake disks or plates. This danger is especially pronounced with synthetic yarns. Yet damage to the braking surfaces worsens the quality of the yarn traveling between them.
These difficulties mean that the yarn brake has to be cleaned, or even replaced completely, at certain time intervals.
In view of these problems, yarn brakes that use brake disks provided with electromagnets acted upon by alternating current have already been contemplated, in German Patent Disclosure DE 30 29 509 A1. These electromagnets are provided in such a way that the brake disk or disks oscillate.
According to DE 30 29 509 A1, difficulties can arise in regulating the alternating current for varying the brake force, yet only relatively narrow control limits for the braking force are available.
In the industry, a yarn brake is known in which two brake plates, each with a central opening, are supported rotatably on a vertically arranged ceramic pin. The ceramic pin is joined rigidly on one end to an electromagnet that is excited by pulsating current. The brake plate remote from the electromagnet is of ferromagnetic steel and is seated on the ceramic pin so as to be axially displaceable. The brake plate is attracted in pulsating fashion by the electromagnet, so that a yarn passed between the brake plates is braked between them.
The brake plate located at the electromagnet rests on a stop. The upper brake plate, since it is acted upon by pulsating forces, executes a slight oscillation, which is disadvantageous with a view to constancy of the braking force exerted on the yarn traveling therethrough.
Another yarn brake is known from U.S. Pat. No. 4,313,578, van Wilson et al. It has two brake plates, seated on a common shaft, one of which is magnetic and the other is nonmagnetic. Coaxially to the shaft carrying the brake plates, there is an electromagnet for the nonmagnetic brake plate; it is capable of attracting the outer, magnetic brake plate toward the nonmagnetic brake plate. A plastic disk of PTFE (teflon) is located between the electromagnet and the nonmagnetic brake plate and is intended to reduce the friction and wear between these parts. The electromagnet is triggered via a control circuit that generates a more or less high current flowing through the electromagnetic, depending on the desired yarn tension. To avoid remanence effects, particularly when the magnetic force generated by the electromagnet decreases, the voltage applied to the electromagnet is pulsed at negative polarity. This is intended to reduce the magnetic force in a desired fashion when the excitation of the electromagnet is decreased.
Action is exerted upon the electromagnet with current pulses solely for demagnetization purposes and to avoid remanence effects. As in the previous example, the magnetic force exerted by the electromagnet also acts only upon the brake plates, so that at least during the reduction in magnetic force by the pulsating voltage, axial oscillation of the brake plates is again brought about, which is deleterious to the constancy of yarn tension.
Finally, U.S. Pat. No. 5,343,983, Horvath et al., discloses a yarn brake in which two brake disks, having a central opening, are retained in a bearing means and are urged resiliently toward one another. In a series of embodiments, the brake disks are rotatably supported with radial play in the bearing means. Causing the bearing means to oscillate radially, oscillation creates a torque that drives the brake disks, by way of the existing play between the brake disks and the bearing means. In another series of embodiments, the radial oscillation generated by an oscillatory motion generating means is carried directly to the brake disks. In both models, both the bearing means and the brake disks oscillate radially, thereby bringing about not only the aforementioned driving effect but also a cleaning effect of the brake disks.